Nine Inch Nails, 'Ghosts I–IV' (NIN.com)

Since leaving Interscope last year, Trent Reznor has been on a productive tear. The latest evidence: For as little $5, you can download Ghosts IIV (or for you diehards, Halo 26), a stunning collection of 36 instrumental tracks that is one of the most varied and ambitious releases of his career. Highlights include "26 Ghosts III," which, like the best Nine Inch Nails instrumentals (think "Just Like You Imagined" from 1999's The Fragile), is a brutal, tension-building groove that fades away elegantly. But "31 Ghosts IV" is something different, as chugging synths suddenly give way to nimble Eddie Van Halenlike wizardry. Could Reznor also be diversifying into the Guitar Hero market?